Originally published in the Fullerton College Hornet, Vol. 78, Issue 23; 28 Apr 1999
Though identical in size, DVDs can hold up to 25 times more data than a CD, leading one to wonder, “how do they cram all that graham?”
The CD, or Compact Disc, was developed by Phillips and Sony in 1981 as a new medium for stereo music recordings.
DVDs and CDs can easily be mistaken at first glance. They are both 120 millimeters in diameter and 1.2 millimeters thick. Both rely on lasers to read data stored in pits in a spiral track. Beyond this, the two are quite different.
On a CD, the space between the tracks is only 1.6 microns and the pits range from 0.83 to 3.0 microns long. A micron is one thousandth of a millimeter.
After the pits are stamped into a plastic disk, it is then coated with aluminum, giving the disk its characteristic silver color. The aluminum is then coated with a thin layer of lacquer, which provides a smooth surface on which the disk’s label is printed.
A CD can hold up to 74 minutes of audio, or about 680 megabytes of data. Since the data is evenly spaced in the track, the laser will read more data in a single revolution at the outer edge than the center. Therefore, the disk needs to spin faster at the center than the outer edge to maintain a constant linear velocity, or CLV.
Originally, DVD stood for Digital Video Disk, as it was intended to replace videotapes. A little insight and creativity led to a host of other applications, so the name was changed to Digital Versatile Disk. Fortunately, nearly everyone simply calls it DVD.
Where CDs are adequate for music or most computer applications, full-length movies and large programs that span multiple CDs are too large and require a faster CLV than is available on a CD.
Major differences between CD and DVD drives are the spacing of the pits in a track, the width of the track itself and the wavelength of the laser that actually reads the disk.
On a DVD, the width of a track is 0.74 micron and the smallest data pit is 0.40 micron. The laser’s wavelength was reduced from 780 nanometers to 640 nanometers.
The result is a disk with a capacity of 4.7 gigabytes, but since the disk needs to be only half as thick as a CD, a blank platter can be glued on top to create the necessary depth. However, a second track can be placed on the platter to give a 9.4 gigabyte capacity. Also available is an additional track on each platter, allowing a maximum 17 gigabyte capacity per DVD.